1. Field of the Invention
The present invention relates to gate valves, and more particularly to a boltless bonnet structure for gate valves.
2. Description of Related Art
Heretofore, various designs have been used and proposed for use in fastening a gate valve bonnet to the valve body. Traditionally, a bonnet has been fastened by providing the valve body and the bonnet with complementary bolt-through flanges and a plurality of bolt and nut fasteners extending therethrough.
More recently, certain designs have come forth wherein the bonnet is fastened without the use of bolts. Examples of such designs can be found in U.S. Pat. No. 4,431,163, in the name of Pierre Barbe and assigned on its face to Pont-a-Mousson, S. A. and a further design employed in a gate valve marketed by Pont-a-Mousson under the trademark "EURO-20". Both of these valves avoid the problems, primarily corrosion-related, associated with the use of bolted connections for gate valve bonnets. These bonnet designs, however, both suffer from certain drawbacks which detract from their desirability for use.
The design disclosed in the Barbe patent requires complementary grooves to be provided in mating edges of the bonnet or cover, and the upper housing of the valve into which cylindrical locking pins are inserted. This design enables access to the valve stem upper seals without danger of the bonnet dropping into the valve body; however, when removal of the bonnet is desired to gain access to the interior of the valve body, the locking pins must first be removed. This may be a most difficult task, particularly when the valve is installed in a line.
The EURO-20 gate valve design employs a bonnet-to-valve body connection which provides threaded members which are more easily connected and disconnected. That design suffers the disadvantage that in order to gain access to the valve stem upper O-ring seals, components must be removed which leave the bonnet in danger of coming unseated, and dropping out of position into the interior of the valve body.
One further example of a "boltless" bonnet is disclosed in U.S. Pat. No. 4,699,359, in the name of Walfried David, and assigned on its face to Bopp & Reuther, GmbH. The gate valve in this patent employs a threaded-type connection for the bonnet structure wherein a supporting ring having two supporting legs extending downwardly to the valve body is clamped in place by a collar on a locking screw. The bonnet structure in this patent suffers from the same disadvantage as the aforementioned EURO-20 valve in that the bonnet could unseat and drop when an attempt is made to gain access to the upper part of the valve stem.
It is therefore an important object of the present invention to provide a gate valve having a boltless bonnet connecting structure which substantially eliminates the possibility of having the bonnet drop while accessing the upper portion of the valve stem.
It is a further object of the present invention to provide a bonnet having threaded connections for fixing the bonnet in place within the valve body which also provides isolation of the threads from corrosive environments.
It is another object of the present invention to provide a gate valve design wherein the upper valve stem O-ring seals are accessible for maintenance without disrupting the integrity of the valve body-to-bonnet seal.